A common problem in medical devices intended for blood contact is the biocompatibility of the surface of such devices. Medical devices such as artificial heart valves are often in permanent or at least long-lasting contact with blood. This does not only apply to medical devices implanted or otherwise introduced into the human or animal body, but also to medical devices used in extracorporeal systems like a heart-lung machine. If no special care is taken, the contact between the medical device and the blood may result in so-called "clotting" (coagulation) at the surface of the medical device. Such clots may render the medical device (e.g. a sensor) inoperable. Clots can also reduce the free cross-sectional area of a blood vessel, therefore reducing blood flow. Perhaps most dangerously, a clot formed on the surface of the medical device may be detached by flowing blood and be transported by the blood flow into a position where it can occlude a blood vessel, (in particular, a capillary) thus causing thrombosis.
The situation is even more critical in case of a catheter or an intravascular blood gas sensor introduced in a blood vessel of relatively small diameter such as the radial artery or the femoral artery. The catheter may be completely blocked by a clot, so that the blood pressure cannot be measured or that no blood samples can be taken. In case of an intravascular blood sensor, the active area may be blocked, and no fresh blood can reach the sensor. The above considerations are of particular importance when long-term contact between the medical device and the blood is intended. Even with optimum material selection for the medical device, clotting cannot be reliably prevented.
A common approach to solve this problem, i.e. to prevent the formation of clots, is to coat the medical devices with a biological coating (sometimes also referred to as bioactive or antithrombogenic coating). Coatings suited for this purposes are well-known in the art. For example, a heparin-based coating, such as described in U.S. Pat. No. 4,810,784, may be used. Other suitable biocompatible materials are e.g. phosphorylcholine (EP-B-157 469) or polyester (U.S. Pat. No. 4,792,599). Hirudin may be used as well. Other biological coating materials useful as anticoagulants are known in the art.
A common problem when applying such biological coatings to a medical device is to ensure reliable adhesion between the coating and the surface of the medical device, i.e. reliable immobilization-of the coating. It is understood that poor adhesion would lead to detachment of the coating so that the medical device loses its antithrombogenic properties. As the biological coating does not adhere to the surface of the medical device by itself, additional measures have to be taken. Further, it has to be ensured that the biological coating does not lose its bioactive properties during the immobilization process.
A known solution to this problem is to coat the surface of the medical device with a polymer and to apply the biological coating to the polymerized surface. For this purpose, the uncoated medical device is put into a polymer bath, i.e. a solvent containing dissolved polymer. When the medical device is removed, its surface carries a thin film of solvent containing the polymer. The solvent then vaporizes, such that the pure polymer resides on the surface of the medical device. Subsequently, the biological coating is applied, e.g. by putting the medical device into an appropriate bath. However, the immobilization of a biological coating fastened on the surface of a medical device in this manner is not always reliable.
The inventor in the present case has particularly noted that parts of the biological coating detached in use from an intervascular blood gas sensor. This has particularly happened when a medical device is stored or deposited in a liquid for a longer time period (e.g. an intravascular sensor requiring a wet or liquid environment to keep its operability even when not in use). Such detachment is an intolerable disadvantage of the known technique, partially because of the danger for the patient as blood clots may attach to the uncoated portions of the surface, and partially as such removal of the biological coating may affect the measuring accuracy of the sensor where parts of it are coated and others are not.